Stimuli‐Responsive Sponge for Imaging and Measuring Weak Compression Stresses

Abstract Imaging and measuring compression stresses secure a safe and healthy life. Compression stresses in kPa range are not easily detected by conventional mechanoresponsive materials because microscopic molecular motion of the chromophores is not induced by such weak stresses. Moreover, imaging of the stress distribution is not achieved so far. The present study shows a sponge device combining two stimuli‐responsive materials, a capsule releasing interior liquid and color‐changing polymer in responses to compression stress and chemical stimulus, respectively. The stimuli‐responsive capsule is dispersed on a melamine sponge comprised of the fibers with coating the layered polydiacetylene (PDA). The application of weak compression stresses induces collapse of the capsules, outflow of the interior liquid, and subsequent irreversible color change of PDA. The cascading response in the sponge device colorimetrically enables imaging of the distribution and measuring the strength of the compression stresses in kPa range. Furthermore, the device demonstrates imaging and measuring unknown weak compression stresses applied by the irregular‐shaped objects. A couple of clinical issues in surgical operation of intestine are studied using the stress‐imaging sponge device. The device and its design strategy can be applied to stress imaging in a variety of fields.

P. S3 × 5 mm for 541 and 675 kPa. The contact area was determined by the size of the device. After the compression stresses were applied for 10 s, the remaining DL was immediately removed with air blow. The sponge device was cut by tweezers to prepare the cross section. The photograph was taken using a smart phone (iPhone 13) at 100 s after releasing the compression stress. The red-color intensity (x) values were calculated from the RGB values of the cross sections (10 × 5 mm) using a software for image analysis (Image J) according to Eqs, S1 and S2. [S1] The successive processes were completed within total 120-150 s. The data including the mean and standard deviation were obtained using three different sponge devices to ensure the reproducibility. In the present work, the thickness of the device was constant at 5 mm. As ∆x value is an increment of the ratio of the red-color domain to the original state (∆x = x − x0), the relationship between P and ∆x in Figure 3e is used regardless of the lateral size in the cross section of the device.

Scheme 1.
Schematic illustration of the procedure and time course from the device preparation of PDA/DL, compression, removal of the remaining DLs, cutting of the sponge to prepare the cross section, and taking photographs for the readout ∆x.

P. S5
Previous works about detection of compression stresses Figure S1. Detection ranges of the compression-stress sensing materials in previous works.
The symbols A-U correspond to the references [21-41], respectively.

P. S6
Structural data of melamine sponge and PDA coating Figure S2. Pore-size distribution (a) and UV-Vis transmittance spectrum (b) of a commercial melamine sponge used in the present work. The average size and distribution of the pore were estimated from the SEM images. The average width of the fibers in the network was 4.72 ± 1.82 μm. The melamine sponge (20 × 15 × 5 mm) contained average 17 wt% of PDA to its own weight (Table S1). UV light permeated the melamine sponge (20 × 10 × 2.5 mm) ca. 5 % at 254 nm ( Figure S2b). The thickness of the sponge was reduced to half. As UV light was irradiated to both the surface and back sides of the sponge, UV transmission to 2.5 mm in depth supports the irradiation to whole of the sponge.  S3c,d), even though DLs smaller than 125 μm and lager than 250 μm were included after the sieving. The smaller and larger DLs were removed by the particle-size selection (the colored data in Table S2).
Water as the solvent of the interior liquid in DL was gradually evaporated within ca. 20 min at room temperature under ambient pressure ( Figure S3e). On the other hand, the resultant DL was stored in a plastic bottle after the preparation until the use. When the PDA/DL sponge device was prepared, a specific amount of DL was taken from the storage bottle. The disruption of DLs with the evaporation of water was not observed in a plastic bottle. In contrast, DL stored in a glass bottle caused the disruption with evaporation of water. When water evaporated from DL forms the liquid film with wetting on the hydrophilic surface of glass, the interior liquid as the smaller water droplets diminishes by Ostwald ripening. DLs are preserved in a plastic bottle because the hydrophobic wall prevents water from wetting on the wall. In this manner, the stored DL in a plastic bottle can be used anytime.

P. S9
Structure characterization of the color-changed device Whereas the original PDA on the melamine sponge showed the absorption peak corresponding to dimerized carboxy groups of C=O around 1690 cm −1 , the absorption peak corresponding to P. S10 carboxylate groups of C=O appeared around 1650 cm −1 after the disruption of DL ( Figure S4a).
The peak characteristic to the layered structure at 2θ = 1.90 ° (d = 4.64 nm) was weakened and shifted to 2θ = 1.45 ° (d = 6.09 nm). An increase in the d value is consistent with that of the PEI-intercalated layered PDA in our previous work. [S2] SEM image and EDX analysis indicate the presence of nitrogen (N) and silicon (Si) originating from the interior PEI and shell SiO2 particles in the red-colored domain, respectively ( Figure S4c,e). In contrast, these elements were not detected in the blue-colored domain ( Figure   S4d,f). These facts indicate infiltration of the fragmented SiO2 particles and interior PEI in the sponge.
Additional Reference After applying stress, the remaining DL was removed from the sponge device. The device was cut by scissors to obtain the cross section for taking the photographs. When the DL was not removed, the cutting caused the further disruption leading to the color change ( Figure S5).
Therefore, the remaining DLs were removed by air blow. A commercial small handy blower for cleaning of camera and lens was used to this removal process. Propellant or high-pressure gas are not required. In addition, tr has no influence on Δx.
P. S12 Photographs of the compressed sponge device Figure S6. Cross-sectional photographs of the PDA/DL sponge device with the application of the compression stresses on three different devices (i)-(iii) using a tester.   The responsivity was improved using the more elastic melamine sponge B compared with the sponge A. As the sponge B is strained by the weaker compression stress, the diffusion distance of PEI into the sponge is shortened.
P. S14 P. S15 The responsivity was improved with an increase in the amount of the loaded DL. As the number of collapsed DLs and volume of the released PEI increases, the responsivity is improved.

Sensitivity tuning by changes in the loaded
P. S16 The PDA/DL device showed the color changes in response to tc. As the diffusion length of the interior liquid increased with increasing tc, the larger ∆x is observed.

Response to applied duration of the compression stresses
P. S17 Relationship between P and ∆x.

Response of DLs introduced in the inside
The DL was introduced in the sponge with shaking ( Figure S8a). In this reference experiment, the particle size of DLs was changed from 125-250 μm (normal condition) to 53-125 μm to introduce in the inside of the sponge. When the PDA sponge including the DLs in the inside of the sponge, the colorimetric response was not observed (Figure S9b,c). However, the detailed reasons why DLs in the inside of the sponge device are not disrupted in response to the stress are still unclear. We are now studying the detailed disruption mechanisms of DLs.
P. S18  The color-changed depth (Lred) increased with an increase in P ( Figure S9b and Table S9). Lred was measured on the cross-sectional photographs. The increment (∆x) of the reference device using c-DL/PEI was larger than that using c-DL ( Figure S12c). The results indicate that the diffusion of PEI contributes to an increase in Lred improving the sensitivity. Moreover, the sensitivity was enhanced by the color changes of PDA ( Figure S12d).   When the softness model III was compressed and stapled using a linear stapler device, as shown in Figure 6a, the formability of the staples, i.e. the shape, was different ( Figure S14a). Formation of B-shaped staple is required to achieve adequate stapled state ((i) in Figure S14b). On the other hand, the hard and/or thick tract causes the inadequate stapled states ((ii)-(iv) in Figure   S14b). Therefore, the studies on the compression stresses applied to the tracts with the different softness and thickness are significant to avoid the inadequate stapling.

P. S24
Sensing the softness models I-III Figure S15. Softness of the models I-III and their sensing results. (a) Stress-strain curves of the PDA/DL device set between the two softness models, as shown in Figure 6b. (b) Crosssectional photographs of the models I-III. The stress-strain curves in Figure 6c were measured using only the softness models I-III. Figure   S11a shows the stress-strain curves of the models I-III with the PDA/DL device for the sensing experiments. The melamine sponge B with dispersion of DL 25 mg was used for this experiment ( Figures S7 and S8).

P. S25
Stress-distribution imaging using a circular stapler Figure S16.
Cross-sectional photographs of the PDA/DL sponge device for stressdistribution imaging of the compression using automated anastomosis device with circularly arranged staples.
The profile in Figure 6k,l was prepared using the average and standard deviation based on these eight photographs.